The Southern Ocean and Climate:What did we learn during WOCE?

Steve Rintoul

CSIRO Marine Research and Antarctic CRC

Australia

Pre-WOCE view of the ACC/SO

• 2 circumpolar fronts• wind-driven, in (flat-bottom) Sverdrup balance• bottom form stress balances wind?• Drake Passage transport = 134±13 Sv• transport variability is barotropic• no net meridional flow through Drake Passage gap • poleward eddy heat flux in Drake Passage, SE NZ• zonal circulation independent of meridional

circulation• water masses exported to lower latitudes, but rates

and mechanisms unknown

Progress in the “WOCE era”

• remote sensing (SST, SSH)• new instruments (e.g. ALACE floats)• observations outside of Drake Passage• improved model realism/resolution/diagnostics• air-sea flux estimates from reanalyses• advances in dynamical understanding

Orsi, 2002

10,000 stations south of 25S since 1990

Oxygen on 27.4

4-year mean SST gradient from ATSR reveals multiple filaments and branches, which merge and split.

Rintoul, Hughes and Olbers 2001

Tracking ACC fronts using satellite altimetry

Careful comparison of hydrography and absolute sea surface height maps shows each frontal branch corresponds to a particular SSH contour.

We can use altimetry to track fronts, every 10 days since 1992.

Sokolov and Rintoul, JMS, 2002

•SAF: 3 branches, merge near 140E, eddy-rich downstream of change in orientation of SEIR.

•PF: 2 branches, separated by >500 km at SR3, merge after crossing ridge crest.

•PF, SACCF: strong equatorward deflection over ridge.

•Narrow meander envelopes near ridge.

ACC Transport

Repeat sections showheat transport south ofAustralia varies by 0.6 x 1015 W (relative to0C).

Variability is large (e.g.relative to north-southheat flux in Indian andPacific.)

Climate impact?Rintoul and Sokolov, JGR, 2001

Cunningham et al., JGR, 2002

Drake Passage transport: 1368.5 Sv

ACC transport

500 billion Lone Stars/sec

www.mylifeisbeer.com

Rintoul and Sokolov, 2001; Cunningham et al., JGR, 2002

ACC transport in neutral density layersAustralia (SR3) color; Drake Passage (SR1) black

The tight relationship between temperature at 650 m and the baroclinic transport streamfunction can be used to determine transport (above 2500 m) from temperature msmts. alone.

Rintoul, Sokolov and Church, JGR, 2002

Net baroclinic transport time series from XBT data(squares) and CTD data (diamonds)

Net baroclinic transport south of Australia (1993-2000)

Transport estimated from altimeter (thin line), low-passed (thick blue line).

Empirical relationshipbetween surface heightand transportfn used to estimate transport.

Continuous record fromaltimeter shows XBTtime series is aliased.

Rintoul, Sokolov, Church, 2002

“Streamwise” average of absolute velocity of Subantarctic Front: Total transport = 116 Sv; barotropic = 16 Sv.

Phillips and Rintoul, JPO, 2002

Eddy heat flux

Poleward eddy heat flux across SAF south of Australia is larger than previously measured elsewhere in the Southern Ocean.

Phillips and Rintoul, JPO, 2000

Rintoul, Hughes and Olbers 2001

Bottom pressure torque (color); barotropic streamfn (black)

Is the ACC in Sverdup balance?

ßx = pb H + + F

-fV1 = - '1p'1x + o - R1

-fV2 = '1p'1x - '2p'2x - R2

-fV3 = '2p'2x - hpbx - R3

V = net meridional volume flux o = wind stress = layer thickness p = pressure R = Reynolds stress divergence pb = bottom pressure

Steady, zonally-integrated momentum balance:

Surface (includes Ekman)

“unblocked” layer

“blocked” layer

1

2

3

V1 = - o/f

V2 = 0

V3 = hpbx /f = o/f

Overall balance of zonal momentum is between wind stress and bottom form stress.

No interfacial form stress:

Ekman transport in surface layer

No transport in “unblocked” layer

Deep geostrophic flow balances Ekman

Adding the three equations and using fact thatmass is conserved ((Vi) = 0):

o = hpbx

Again, overall balance of zonal momentum is between wind stress and bottom form stress.

Interfacial form stress 0:

o = 'ip'ix= hpbx

Wind stress = interfacial form stress = bottom form stress

Note that both standing and transient eddies contribute to interfacial form stress.

Adiabatic flow (Vi = 0):

z('ip'ix) 0

Mixing and surface buoyancy fluxes drive mass exchangebetween layers, so Vi = net diapycnal exchange 0.

Diabatic flow (Vi 0):

Divergence of interfacial form stress drives meridional flow in the unblocked layer.

Buoyancy forcing, eddy stresses, and meridional flow are intimately linked to the zonal momentum balance.

What controls the transport of the ACC?

Observations and a variety of models suggest ACC transport is a function of: n (n = 0-1?) x – buoyancy flux– topographic interactions– baroclinic instability / eddy fluxes

(Gent, Tansley, D. Marshall, J. Marshall, Karsten, Olbers, Rintoul, Sokolov, Gille, Gnanadesikan, Hallberg, …)

Schmitz (1996)

Orsi et al., 1999

Orsi et al., JGR, 2002

CFC inventory: 8 Sv AABW; 21 Sv total input to deep ocean

SO Overturning

By including the water mass transformations driven by air-sea fluxes, we can quantify the overturning circulation for the first time.

• vigorous deep cell

• weak upwelling through the thermocline

• NADW global cell closed by DW IW conversion in SO

Speer et al., 2000; Sloyan and Rintoul, JPO, 2001

34

52

46

42

eddy mass flux

Models also suggest the NADW overturning cell is closed by upwelling and water mass transformation in the SO.

Döös and Coward (1997)

2530

10

13

4

88

Formation, circulation and consumption of intermediate and thermocline waters.

Sloyan and Rintoul (2001)

11

Speich et al., GRL, 2001

Upper branch of the global OTC

“cold” = 6.5 Sv

“warm” = 5.3 Sv

“cool” = 3.1 Sv

Wong et al., 1999

Intermediate depth waters in both hemispheres have become fresher in recent decades.

Banks et al., GRL, 2000

Climate models show similar response; suggest strongest ocean climate change signal in SO.

Rintoul and England, JPO, 2002

Observations south of Australia show large variability in mode water properties from year-to-year, driven by changes in cross-frontal Ekman transport (not air-sea fluxes).

Circles show T-S properties of SAMW south of Tasmania;size of dot is proportional to strength of mode.Triangles and squares are data from 1968 and 1978.

Warming of the Southern Ocean

Gille, Science, 2002

Warming of Weddell Sea Warm Deep Water

Warm Deep Water flowing into and out of the Weddell Sea has warmed by about 0.3C since the mid-1970’s.

(Robertson et al., 2002)

Climate models suggest SO overturning will slow down as a result of global warming.

Warming and fresheningincreases the high latitude stratification,shutting down AABWformation.

Is this result realistic?Can we observe the change in stratification?

Hirst (1999)

The Southern Ocean is the largest zonally-integrated sink of anthropogenic CO2.

Sabine et al., 2002

Massom et al., 2001

Thompson and Solomon, Science, 2002

Southern Annular Mode/Antarctic Oscillation

Antarctic Circumpolar Wave

White and Peterson, 1996

Air temperature Sea ice extent

SLP: El Nino SLP: La Nina

Antarctic Dipole

Subtracting May composites for El Nino and La Nina events reveals the impact of ENSO on the Southern Ocean.

Response consists of a dipole with centres in the Atlantic and Pacific sectors, driven by the PSA teleconnection.

(Yuan, 2001).

Modes of variability:• local or remote forcing?

• ocean response?

• feedback?

• coupled?

• regional climate impact?

New view of the ACC/SO

• multiple filaments, which split and merge

• bottom pressure torque important (i.e. not in flat-bottom Sverdrup balance)

• transport = f (, x, buoyancy forcing, topography)

• zonal and meridional circulations intimately linked

• eddies carry mass and heat poleward across Drake Passage gap

• quantified rate and mechanisms of water mass formation

• water mass transformation in SO closes overturning cells

• observed change at all depths

• identified modes of variability

Science questions

Strength, variability and sensitivity of SO overturning?

Dynamics and climate impact of SH atmosphere, ocean, ice variability?

How much mixing takes place in the Southern Ocean?

Does the SO gain or lose heat and freshwater?

Impact of SO variability (low latitudes, regional climate, global overturning)?

Conclusions

We have made remarkable progress in understanding the Southern Ocean during the “WOCE era.”

The Southern Ocean strongly influences regional and global climate, and is sensitive to change.

The prospects for further progress are good. We can now identify specific hypotheses and design observing systems and models to test them.

A similar relationship can be used to determine transport for satellite measurements of sea surface height.

Relationship between surface dynamic height and transportfunction, determined from the 6 CTD sections.

A test of how well baroclinic transportcan be estimated fromaltimeter data.

Residuals are typically small (less than 5 Sv).

Demonstrates most ofaltimeter signal is dueto changes in baroclinicstructure above 2500 m.